Conventionally, a single-component development method only using toner as a developer and a two-component development method using toner and carrier are known as development methods in image forming apparatuses using electrophotographic methods.
In such a single-component development method, toner is commonly passed through a regulation section formed by a toner carrier and a regulation plate pressed against the toner carrier, thereby the toner is charged and a desired toner thin layer can be obtained, resulting in advantages in simplification, miniaturization, and cost reduction of an apparatus.
However, toner deterioration can be easily accelerated due to strong stress caused by such a regulation section, and the charge acceptance of toner can be easily decreased. Further, a regulation member as a charge providing member for a toner and the surface of a toner carrier are contaminated with toner or external additives, whereby charge providing properties for the toner is decreased, whereby the charge amount on the toner is decreased and problems such as fogging are caused. Thereby, the service life of a development apparatus is usually shortened.
In contrast, in a two-component development method, toner is triboelectrically charged by being mixed with carrier, whereby causing small stress, and the carrier has a strong resistance to the contamination with toner or external additives, since the area of carrier surface is large.
However, in such a two-component development method, when an electrostatic latent image on an image carrier is developed, the image carrier surface is brushed with a magnetic brush formed of developer, resulting in such a problem that magnetic brush traces are generated in a developed image. Further, a carrier is easily allowed to adhere to the image carrier, resulting in the problem of image defects.
A so-called hybrid development method as a development method is proposed (refer to, for example, Unexamined Japanese Patent Application Publication No. 59-172662) to solve such an image defect problem and to realize high image quality comparable to that of a single-component development method while the service life is as long as a two-component development method using a two-component developer, in which hybrid development method a two-component developer is supported on a developer carrier and only toner is supplied from the two-component developer to a toner carrier for development.
However, in such a hybrid development method, when an image is formed at a high speed, the flying of toner is not short enough for a shorter development nip time, resulting in such a problem that image density is decreased.
The above problem is in common with noncontact single-component development. However, it has not seen as a serious problem, since it has been used only in a slow speed region to avoid a problem of heat generation at a regulation section or a problem of toner fusion.
In hybrid development, these problems do not exist, whereby image formation can be carried out at a substantially high speed. However, for example, in an apparatus having a system speed of more than 500 mm/s, there is a possibility that the above problems are produced.
As a countermeasure against the density decrease at such a high speed of development, a method is known, in which a plurality of toner carriers are provided to lengthen the development time for toner flying to ensure toner density (for example, refer to Unexamined Japanese Patent Application Publication No. 2005-37523).
In this configuration, even when a photoreceptor is rotated at a high speed, due to the existence of a plurality of toner carriers, a toner can be flown more than once, whereby the nip width to form a toner image on the photoreceptor is increased, resulting in an advantage to inhibit the density decrease of the toner image associated with higher speed production.
In Unexamined Japanese Patent Application Publication No. 2005-37523, used is an image forming process in which an electrostatic latent image is formed, on an image having been developed on an image carrier, to be developed with different color toner, whereby a plurality of toner images are superimposed on the image carrier. Therefore, it is important that a toner image formed on the upstream side is not disturbed. In order to control toner reciprocation in the development nip and to ensure adequate toner density, emphasis is made on the utilization of a development nip width increased by using a plurality of toner carriers. It is disclosed that it is desirable to further enhance the development capability of a toner carrier of the downstream side than that of a toner carrier of the upstream side in the image carrier rotating direction, in order to realize the above object.
On the other hand, an image forming process is known, in which there is an image forming process to form an image of a plurality of colors, in which process a plurality of steps to transfer a toner image, obtained by developing an electrostatic latent image on an image carrier, onto a recording medium such as an intermediate transfer body or paper are performed.
In this manner, when no toner image is not formed on the upstream side d, it is desirable that toner reciprocation at the development nips is made to be more active and a toner is caused to actively reciprocate in an increased development nip width to enhance uniformity of a toner image in the high speed range and reproducibility of fine dots and thin lines.
Further, since a plurality of such toner carriers are provided, a toner image is formed with the upstream side toner carrier, which is on the upstream side in the rotating direction of the image carrier on which an electrostatic latent image is formed, and whose development capability is enhanced, whereby not only a toner on the toner carrier in the development nip of the downstream side but also a toner image formed on the image carrier by the toner carrier of the upstream side join for toner reciprocation, resulting in more vigorous toner reciprocation.
In such a manner, in a configuration provided with a plurality of toner carriers, the development capability of each of the toner carriers is allowed to vary depending on the intended purpose, whereby advantages thereof can efficiently be effective.